netdev - Re: [PATCH net-next v3] net-loopback: set lo dev initial state to UP

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Date:   Tue, 9 Feb 2021 08:23:26 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Petr Machata <petrm@...dia.com>
Cc:     Jian Yang <jianyang.kernel@...il.com>, <davem@...emloft.net>,
        <netdev@...r.kernel.org>, Mahesh Bandewar <maheshb@...gle.com>,
        Jian Yang <jianyang@...gle.com>
Subject: Re: [PATCH net-next v3] net-loopback: set lo dev initial state to
 UP

On Tue, 9 Feb 2021 12:54:59 +0100 Petr Machata wrote:
> Jian Yang <jianyang.kernel@...il.com> writes:
> 
> > From: Jian Yang <jianyang@...gle.com>
> >
> > Traditionally loopback devices come up with initial state as DOWN for
> > any new network-namespace. This would mean that anyone needing this
> > device would have to bring this UP by issuing something like 'ip link
> > set lo up'. This can be avoided if the initial state is set as UP.  
> 
> This will break user scripts, and it fact breaks kernel's very own
> selftest. We currently have this internally:
> 
>     diff --git a/tools/testing/selftests/net/fib_nexthops.sh b/tools/testing/selftests/net/fib_nexthops.sh
>     index 4c7d33618437..bf8ed24ab3ba 100755
>     --- a/tools/testing/selftests/net/fib_nexthops.sh
>     +++ b/tools/testing/selftests/net/fib_nexthops.sh
>     @@ -121,8 +121,6 @@ create_ns()
>      	set -e
>      	ip netns add ${n}
>      	ip netns set ${n} $((nsid++))
>     -	ip -netns ${n} addr add 127.0.0.1/8 dev lo
>     -	ip -netns ${n} link set lo up
> 
>      	ip netns exec ${n} sysctl -qw net.ipv4.ip_forward=1
>      	ip netns exec ${n} sysctl -qw net.ipv4.fib_multipath_use_neigh=1
> 
> This now fails because the ip commands are run within a "set -e" block,
> and kernel rejects addition of a duplicate address.

Thanks for the report, could you send a revert with this explanation?